The legal minimum tread depth for car tyres in European is 1.6mm throughout a
continuous band comprising of the central three quarters of the breadth of tread
and round the entire outer circumference of the tyre.
Most car tyres have tread wear indicators, usually at least six small ribs across
the bottom of the main tread grooves, and when the tread surface becomes level
with these ribs the tyre is at the legal limit and must be replaced.
Simple and cheap gauges are widely available to give an acceptable guide to tread depth.
One of the features of tyre tread pattern design is to give good grip in wet conditions. Generally speaking, available grip reduces as the tyre wears or as the road surface water depth increases.
You should reduce speed in such conditions.
Nicks Tyres is in Brentwood and Chelmsford and is my local tyre shop.
They are very helpful so if you live in the area, check them out for their services.
Tread Wear Indicators
Here is a nearly new tyre. Inside the tread, across the middle 75%,
dotted around the circumference of the tyre are raised indicators.
And this is an old tyre. You can see that the tread wear indicators are flush
with the rest of the tyre face meaning that the tyre is too worn and is illegal.
There are very good online tyre calculators online so I don't think I need my own.